Heretofore, steel plates for, e.g., the body of a car, have been reinforced by various techniques. For example, in the case of the outer panels of car bodies, e.g., roofs, fenders, hoods, trunks, quater panels, and doors, which are in a relatively broad and flat form, and are reduced in thickness, a plate comprising a metallic reinforcing material has been bonded to the inside thereof by spot welding or with adhesives, because it is necessary to provide the outer panels with enough stiffness to withstand external forces. The use of such metallic reinforcing materials, however, gives rise to various problems because of their heavy weights. For example, the weight of the outer panels, which are originally reduced in thickness for the purpose of decreasing the weight of the car body, is increased, the production costs are raised, and furthermore, the step of mounting the outer panels is complicated.
It is also known that polymeric materials, such as asphalt rubber, epoxy resins, acrylic resins, phenol resins, and unsaturated polyester resins, are coated on or bonded to the back surface of the outer panel in a fairly large thickness over a fairly large area for both the purposes of preventing the vibration of car body outer panels and for reinforcing them. It is said that the stiffness is proportional to the cube of the thickness. Therefore, the stiffness of the outer panel can be increased by increasing the thickness of the polymeric material layer. The use of such large amounts of polymeric materials, however, leads to increases in the weight and production costs, as is the case with the above-described metallic reinforcing materials.
Japanese Patent Application (OPI) No. 39966/81 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses a reinforcing material for outer panels of car bodies comprising a thermosetting resin layer (prepreg) and a foamable sheet. Japanese Patent Application (OPI) No. 151361/82 also discloses a reinforcing material comprising a two-layer structure prepreg and a foamable sheet.
However, those prior art reinforcing materials have the following disadvantages:
(1) When the foamable sheet is foamed by heating, the sheet foams three dimensionally and the height of the resulting reinforcing material is not sufficient. Therefore, an excellent reinforcing effect cannot be obtained;
(2) The foamed material after foaming tends to absorb a moisture, resulting in causing corrosion of metallic plates of, e.g., outer panel of car bodies; and
(3) The foamed material leaks out of the edges of the prepreg, resulting in poor appearance.